As a semiconductor device that enables the enhancement of a breakdown voltage and the reduction of on-state resistance, there have been conventionally known the following semiconductor devices: semiconductor devices having a p-n column portion where an n-type semiconductor layer and a p-type semiconductor layer are alternately and adjacently placed, and the devices providing a vertical element using either the n-type semiconductor layer or the p-type semiconductor layer as a drift layer. To ensure a certain breakdown voltage of the whole of such a semiconductor device, a construction has been proposed. In this construction, a semiconductor layer having a lower concentration than that of the same conductivity type semiconductor layer constructing the p-n column portion is provided as a peripheral portion positioned in the vicinity of the p-n column portion. (Refer to JP-B1-3743395 corresponding to U.S. Pat. No. 7,037,789 and JP-A-2003-7676.) By providing the low-concentration semiconductor layer as a peripheral portion, as mentioned above, a depletion layer can be widely expanded in the lateral direction.
To widely expand the depletion layer in the lateral direction (i.e., the direction of juxtaposition of n-type semiconductor layers and p-type semiconductor layers) when the device is off, it is required to ensure a charge balance. With this taken into account, p-n column portions are so set that n-type semiconductor layers and p-type semiconductor layers are equal to each other in impurity amount. However, constructions provided with a low-concentration semiconductor layer as a peripheral portion involve difficulties. Since the low-concentration semiconductor layer exists, a charge balance can be accordingly disrupted in proximity to an end of the p-n column portion, so that the breakdown voltage is reduced.
Thus, it is required for the semiconductor device to improve the breakdown voltage of the entire device.